1. Field of the Invention
The present invention relates to testing and switching systems for information transmission and communications networks and systems, and the like, and more particularly to testing and switching stations therefor which include programmable control and processing circuitry. The present invention also relates to methods and apparatus for measuring network and system parameters including signal level, phase jitter, and noise.
2. Description of the Prior Art
Testing systems for information and communications networks and systems are known as are methods and apparatus for performing certain tests therefor. It is known to place test equipment remotely from a central station and to activate and/or control the test equipment remotely by use of the line normally used for information transmission or communication and by another control line, for example, a commercial telephone line or a DDD line (direct distance dialed network of the Bell system). See, for example, the following U.S. Pat. Nos. 3,431,369; 3,822,367; 3,859,468; 3,860,769; 3,864,533; 3,457,373; 3,814,840; 3,943,305; 3,922,508; 3,912,882; 3,407,877; 3,819,878; 3,743,938.
However, prior art testing systems are lacking with respect to the capability of remote equipment to operate in essentially or substantially independent modes and perform many functions related to these modes. For example, prior art remote test equipment is unable to originate a test communication between the remote equipment and other remote test equipment or the central station or even the same remote test equipment, and they are additionally unable to do so over lines or links that are not normally used for information transmission or communication. For example, U.S. Pat. Nos. 3,822,367 and 3,431,369 both disclose the capability in a test system for a central station in a telephone network to communicate with a remote testing station by commercial dial lines or DDD lines which are not normally used for information transmission or communication wherein the central station originates and controls the communication. An undated publication and a publication dated January, 1976 of ADC Telecommunications division of Magnetic Controls Company also indicate the existence of this capability. Prior art test systems include remote test equipment slaved to the central station which central station usually includes processing and control means for the test system. Accordingly, known remote test equipment is unable to perform many functions as mentioned hereinbefore, one of them being to originate a communication as described hereinbefore. While it is known in the prior art, for example as disclosed in U.S. Pat. 3,431,369, for remote test equipment to cooperate in the performance of system tests and also for the remote test equipment to participate in testing by generating test signals and receiving test signals generated from the central station, such cooperation, generating of test signals and receiving of test signals is completely under the control of the central station which performs the processing. For example, it is well known to control switching at remote locations from a central station to provide a loop-back in which a pair of information transmission or communication lines are interconnected at the remote location. It is also known to do this by interconnecting at the remote location a line normally used for information transmission or communication and a control line, for example, a commercial dial line or DDD line. Additionally, processing and interpretation of the test results is accomplished at the central station, the remote equipment merely relaying information to the central station. Thus, known remote test equipment is unable to perform tests under its own control, i.e., for example, under the control of a program located in the remote equipment and also unable to process and interpret the results of any such testing. Moreover, known remote test equipment is unable to generate, adjust, change, etc., test signals under its own control in accordance with the test at hand and the system parameters in effect. Known remote test equipment is also unable to perform testing between remote locations under control of the remote test equipment. While U.S. Pat. 3,431,369 discloses that far-end unmanned remote test equipment may be controlled through intermediate, unmanned, remote test equipment, both the intermediate and far-end equipment are slaved to the central station and not capable of the independent modes described hereinbefore. It is further not known in the prior art to perform many intensive system tests and to process and interpret the tests results at a remote location during normal information exchange or communication operation of the information transmission and communication network or system. Such intensive tests have heretobefore been performed while the information transmission or communication lines were seized and under control of the testing system.
With respect to the performance of specific tests by the known testing systems, particularly phase jitter tests, signal level and related tests, and noise tests, as mentioned hereinbefore such tests are completely under the control of the central station which also processes and interprets the results. Methods and apparatus for performing these tests are known in the art. A description of particular transmission parameter tests and test specifications applicable to the Bell telephone system is given in AT&T Technical Reference PUB41008 "Transmission Parameters Affecting Voiceband Data Transmission - Description of Parameters" (July, 1974) and in AT&T Technical Reference PUB41009 "Transmission Parameters Affecting Voiceband Data Transmission - Measuring Techniques" (May, 1975). See also U.S. Pat. No. 3,814,868.
With respect to phase jitter, known methods and apparatus for performing phase jitter tests measure the phase angle between a phase-stable test signal and a signal having phase jitter and provide an analog output proportional to the phase difference between the two signals. One such method converts the signals to square waves and provides an analog output proportional in magnitude to the coincidence of the square waves, a zero phase difference between the square waves providing a zero output. Another method employs a variable delay line. Still another method uses a phase-locked loop detector in which the phase detector output has a DC component that is proportional to the phase difference between the signal to be tested and a phase-stable, voltage-controlled oscillator (VCO) signal. These delay line and phase-locked loop techniques have the disadvantage that they are bandwidth-limited and all of the aforementioned techniques require analog-to-digital conversion for use in a digital system. See also U.S. Pat. No. 3,777,256 relating to measurement of frequency delay distortion.
With respect to voltage and current signal level measurements, known techniques in which a digital equivalent of the voltage or current is obtained merely perform an analog-to-digital conversion. The disadvantages associated with such techniques relate to the bandwidth and speed capability of the particular analog-to-digital converter. This is true regardless of whether an instantaneous measurement is performed or whether peak or average measurement are performed. Additionally, the analog-to-digital converter is usually somewhat dependent upon the wave shape of the signal to be measured.
There are several known methods for measuring average power, one well known method being the integration of electrical energy converted to heat energy by a heat sensitive device. A bolometer may be used for this and integrates the energy and provides a DC output proportional to the average power. This method is especially useful at microwave frequencies where discrete measurements are not practical or are impossible. Other power meters average the signal; they are correlated to a particular wave shape and they make approximations with respect to the particular wave shape. Still other power meters use a DC multiplier which multiplies the input signal by itself to give the squared function, and then a capacitor is used at the output, for example, to integrate the power. Usually a long time constant is used to measure the power over, for example, a second. This last type of system makes a "true" power measurement. The major problem in this last type of system, however, is the bandwidth, since an amplifier is used which must be accurate over the bandwidth of the signals being measured in order for a true square to be taken. Additionally, all these methods of measuring power yield a DC value which must be converted from analog to digital for use in a digital processor. One digital technique utilizes an analog-to-digital converter which takes the instantaneous analog value of the input wave, converts it to digital, records the digitized values and then digitally processes the values to determine what the power is. In order to do this, the signal has to be sampled at, for example, 20,000 times in a second and the signal reconstructed according to Nyquist criteria. The 20,000 samples would be the discrete instantaneous amplitude of the wave. These techniques also have the bandwidth disadvantage in that the signal must be within the bandwidth capability of the analog-to-digital converter.
The present invention overcomes these drawbacks and disadvantages of the prior art and realizes additional advantages.